Temperature-Induced Switching of the Mechanism for Intramolecular Energy Transfer in a 2,2‘:6‘,2‘ ‘-Terpyridine-Based Ru(II)−Os(II) Trinuclear Array

Abstract

The synthesis and photophysical properties of a linear 2,2':6',2' '-terpyridine-based trinuclear Ru(II)-Os(II) nanometer-sized array are described. This array comprises two bis(2,2':6',2' '-terpyridine) ruthenium(II) terminals connected via alkoxy-strapped 4,4'-diethynylated biphenylene units to a central bis(2,2':6',2' '-terpyridine) osmium(II) core. The mixed-metal linear array was prepared using the "synthesis at metal" approach, and the Ru(II)-Ru(II) separation is ca. 50 A. Energy transfer occurs with high efficiency from the Ru(II) units to the Os(II) center at all temperatures. Forster-type energy transfer prevails in a glassy matrix at very low temperature, but this is augmented by Dexter-type electron exchange at higher temperatures. This latter process, which is weakly activated, involves long-range superexchange interactions between the metal centers. In fluid solution, a strongly activated process provides for fast energy transfer. Here, a charge-transfer (CT) state localized on the bridge is populated as an intermediate species. The CT triplet does not undergo direct charge recombination to form the ground state but transfers energy, possibly via a second CT state, to the Os(II)-based acceptor. The short tethering strap constrains the geometry of the linker, especially in a glassy matrix, such that low-temperature electron exchange occurs across a particular torsion angle of 37 degrees . The probability of triplet energy transfer depends on temperature but always exceeds 75%.

Electron Exchange in Conformationally Restricted Donor–Spacer–Acceptor Dyads: Angle Dependence and Involvement of Upper‐Lying Excited States

“…This situation can be well explained in terms of activated triplet-energy transfer along the molecular axis. [16] As such, luminescence from the Ru-terpy unit becomes more favourable as the temperature is lowered. It should also be noted that the photophysical properties of the Ru-Ru binuclear complexes [17] are strongly dependent on temperature, because of thermal population of a high-lying, metal-cantered triplet state that is coupled to the ground state.…”

“…We now examine what happens in fluid solution at ambient temperature and emphasize any differences from the behaviour found in a glassy matrix. It should also be noted that electron exchange has been described [16] in a related trinuclear complex built from similar modules.…”

“…At this temperature, it is reasonable to suppose that the strap adopts the lowest-energy conformation and the behaviour found at 170 K tends to confirm the situation already known to exist in the glassy matrix in which k TET is highly dependent on the dihedral angle. [15] The large uncertainty associated with the measured F LUM values and the limited information that be extracted from a single temperature [16] preclude detailed analysis of these results in terms of their exact correlation with f.…”

“…Comparable behaviour was noted previously for the tri-A C H T U N G T R E N N U N G nuclear complex. [16] Before attempting to analyze these data it is worth stressing that triplet-energy transfer is essentially quantitative in all cases and that no other competing process is apparent from the flash photolysis records. That is to say, no intermediate species could be detected.…”

Electronic Energy Transfer to the S2 Level of the Acceptor in Functionalised Boron Dipyrromethene Dyes

“…Indeed, several cases have been described in which the energy donor is the second singlet excited state (S 2 ) of the chromophore, most notably involving carotenoids, [9] metalloporphyrins, [10] azulene [11] or ruthenium(II) polyA C H T U N G T R E N N U N G (pyridine) complexes. [12] Less information is available regarding energy transfer to the corresponding S 2 state of the acceptor, but this situation has been described for energy transfer from pyrene to certain boron dipyrromethene (Bodipy) dyes. [13] Involvement of the upper-lying state is usually evident from computation of the spectral overlap integrals, providing the relevant states are optically bright.…”